In recent years, there has been an increased interest in preventive safety regarding traffic accidents, and as a technology in an automobile drive support system, research and development of an in-vehicle monitoring imaging device have been actively carried out. However, a problem occurs when darkness at night and high-luminance signal light and vehicle light (headlight, and the like) exist at the same time as contrast between light and shade is extremely large and visibility is poor. It is, therefore, necessary to ensure a monitoring performance of an imaging device even in a state where visibility is poor, so as to distinguish the color of a signal and a traffic lane and to ensure color reproducibility consistent with human color vision characteristics.
There is a problem in that in such a scene in which contrast between light and shade is extremely large, a general image sensor exceeds a limit of a dynamic range, and therefore, phenomena referred to as blown-out highlights in which due to over exposure, information of gradation is lost and a pure white image is generated, or conversely, phenomena referred to as blocked-up shadows in which due to under exposure, information of gradation is lost and a pitch black image is generated occur.
Accordingly, an image sensor has been proposed which expands a dynamic range by allowing a relationship between input luminance and an output signal to have a non-linear characteristic.
And in such an image sensor having a non-linear characteristic, a method has been proposed in which after transforming (linearization) an output signal of a region having a non-linear characteristic into a linear signal, various signal processings are performed, and then, a bit width of the signal is narrowed in accordance with a monitor to which an image is outputted, and outputted (for example, Patent Document 1).
On the other hand, IR color camera technology which performs color reproduction having sensitivity to a near-infrared light region by using a color filter which transmits near-infrared light as means for improving a monitoring performance with respect to pedestrians or traffic lanes in darkness has been developed.
Since an IR color camera does not have an infrared removal filter (IR cut filter), it has sensitivity even to light of a near-infrared light region, and by observing near-infrared light emitted from an observed object, or reflected light of near-infrared light emitted by an infrared projector, imaging is performed even in darkness where light from a headlight of a vehicle is not emitted.
An image sensor which can be applied to such an IR color camera has been developed (for example, Patent Documents 2, 3 and 4).